1. Field of the Invention
The present invention relates in general to a communications apparatus and method wherein tapped delay lines and a sliding window correlator are used as a demultiplexer to acquire gigabit per second or higher rate data frames quickly and efficiently with a low synchronization overhead. Although not limited thereto, the invention is particularly suited for use in optical high-speed burst communications.
2. Description of the Background Art
In digital communications techniques, such as optical fiber communications, frames of data are modulated and typically transmitted at frequencies in excess of one gigabit per second. In order to receive such a transmission, a receiver must acquire the transmitted signal before the information may be extracted. Acquiring a signal includes determining the carrier frequency and the bit phase or timing so that the receiver may synchronize with the transmitted signal. In the past, receivers have typically employed a frequency sweep technique in order to acquire the carrier frequency. In the frequency sweep technique, the receiver hypothesizes the correct carrier frequency and searches many frequencies over a predetermined uncertainty range. At each hypothesis, the receiver must also try to acquire bit timing. If the hypothesis fails, the receiver must continue trying to acquire the carrier frequency.
Once the receiver has acquired the carrier frequency, the receiver must then synchronize with the bit phase or timing in the transmitted frame, a process often referred to as clock recovery. In the past, as with carrier frequency acquisition, clock recovery has also typically involved trial and error demodulation of the transmitted signal at the receiver in order to determine where individual bits begin and end. For example, when a particular trial demodulation yields incorrect data, the receiver either advances or retards its approximation to the bit timing and makes another attempt. In the past, therefore, the frequency and bit timing acquisition process often requires substantial time and processing power.
As a result of the foregoing, a receiver cannot typically acquire the carrier frequency and bit timing immediately so that numerous data bits may pass by before the receiver is able to recover information. Thus, to give receivers time to acquire the carrier frequency and bit timing, transmitters typically transmit long preambles or headers of modulated information before the data frame. Although the headers required to allow receivers to acquire the carrier frequency with acceptable probability often introduce an overhead of as much as 30% compared to the actual data in the frame, the use of long preambles is nevertheless acceptable in continuous communications since the preamble length is still small relative to the data stream that follows. However, in burst communications, in which data is transmitted in short segments or bursts, each of which requires carrier frequency and symbol phase acquisition, the use of long preambles is not acceptable since the preambles may well be longer than the data itself. As a result, the long acquisition time associated with resolving both the frequency and the phase uncertainty of the transmitted waveform is incompatible with high-speed burst communications. This is especially true in the case of high-speed burst communications where data rates are in the gigabit per second (Gbps) range or higher.
Another issue presented by burst and other high-speed communications in the Gbps speed range, is the attendant requirement of correspondingly high-speed sampling, clock and other circuitry in the transmitters and receivers that can substantially increase power requirements and costs.
In view of the foregoing, a need remains for an improved signal acquisition technique that can quickly acquire the carrier frequency and bit phase or timing of a signal and is compatible with high-speed burst communications schemes.